[Bug c++/68469] New: warn_unused_result attribute ignored for templates return templates

["dushistov at mail dot ru" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68469

            Bug ID: 68469
           Summary: warn_unused_result attribute ignored for templates
                    return templates
           Product: gcc
           Version: 5.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dushistov at mail dot ru
  Target Milestone: ---

For such code:

#include <functional>

template<typename T>
struct Foo {
        std::function<T()> f(T p)  __attribute__((warn_unused_result)) { return
[p]() { return p; }; }
};

int main()
{
        Foo<int> foo;
        foo.f(5);
}

gcc not produce any warnings, while clang handle this correctly:

g++ -std=c++11 -Wunused-result -c test.cpp
clang++ -std=c++11 -Wunused-result -c test.cpp -o clang_a.out
test.cpp:11:2: warning: ignoring return value of function declared with
warn_unused_result attribute [-Wunused-result]
        foo.f(5);
        ^~~~~ ~
1 warning generated.

return type should depend on template type, like T -> std;:function<T()>,
if replace std::function<T()> with not depend on T type, for example with
`int`, then warning triggers.

I use gcc 5.2